The present invention relates to a method for manufacturing a screw and to a rolling die. In a known method for manufacturing a screw a blank is rolled between two rolling dies for the purpose of forming the screw thread. In this arrangement there is a rolling profile in each rolling die, which rolling profile comprises a host of elongated depressions intended for forming the thread convolutions. Each rolling die comprises a first end and a second end spaced apart from each other in the direction of rolling, wherein a blank during rolling is moved relative to the rolling die from the first end towards the second end.
Conventionally, blanks are used that comprise at least one cylindrical portion that is formed to become the thread. Since during the rolling process as a result of transverse pressure a flow in longitudinal direction of the thread occurs, it is common practice to select the rolling diameter dw0, i.e. the diameter of the blank used, in such a manner that the volume per unit of length in the blank is somewhat greater or equal to that of the finished thread. Thus the following applies to the rolling diameter dw0:dw0=dG0+ddV,wherein dG0 denotes a “cylindrical substitute diameter” of the finish-rolled thread, namely the diameter of an imaginary substitute cylinder whose volume per unit of length corresponds to that of the finish-rolled thread. ddV is an addition to the rolling diameter, which addition is intended to compensate for the axial thrust; typically it is less than 5% of dw0.
If a screw with a desired thread form is to be manufactured in the rolling process, dG0 is determined by this thread form, and ddV results automatically in the rolling process. This means that in order to manufacture a particular thread form in the rolling process, a very specific rolling diameter dw0 needs to be selected; in other words there is no degree of freedom in terms of the selection of the diameter dw0 of the section of the blank on which the thread is to be formed.
In general, an effort will be made to use a simple cylindrical blank because it can be manufactured most simply and cost-effectively; in the present case the diameter of the blank is determined by dw0. However, in practical application this often leads to problems. For example, if a screw head is to be manufactured by pressing a corresponding thread-free section of the blank, the predetermined diameter dw0 is often simply too small for this. In this case it is unavoidable to use a blank with a variable diameter, with a first, slimmer, section for forming the thread, and a second, thicker, section for forming the head. A similar situation occurs in the manufacture of hanger screws, i.e. screws that comprise two different threads that are separate from each other, typically a metric thread and a self-tapping wood-screw thread. For both threads an associated required rolling diameter dw0(1) or dw0(2) results, which diameters, as a rule, will, however, not be identical. In this case, too, it is unavoidable to provide a blank with two sections of different diameters, which leads, however, to a significant increase in the cost of manufacture.